Our work deals with characterization and treatment of metabolic disorders in general, and with disorders of sulphur-containing compounds in particular. We have demonstrated that cystinosis is due to a deficiency of a lysosomal membrane carrier for cystine, and have partially characterized the normal system for cystine transport. The mechanism of action of cysteamine in depleting cells of cystine was shown to be by participating in a disulfide interchange reaction with cystine. Heterozygote status for cystinosis has been ascertained by using a 3H-cystine counter-transport test. We are seeking to determine what small molecules are carried across the lysosomal membrane, and what is the basic defect in lysosomal diseases involving the storage of certain free compounds. Currently, the treatment of cystinosis with pantethine, homocystinuria with betaine, and type III hyperlipidemia with cysteamine are being pursued.